There is an important new paper that has appeared which adds significantly to the understanding of the role of urban areas within the climate system.. The paper is
Jin Menglin, J. Marshall Shepherd and Christa Peters-Lidard, 2007: Development of a parameterization for simulating the urban temperature hazard using satellite observations in climate model, Nat Hazards DOI 10.1007/s11069-007-9117-2 [subscription required].
The abstract reads,
“Abstract Urban surface temperature is hazardously higher than surrounding regions (so-called urban heat island effect UHI). Accurately simulating urbanization-induced temperature hazard is critical for realistically representing urban regions in the land surface- atmosphere climate system. However, inclusion of urban landscapes in regional or global climate models has been overlooked due to the coarse spatial resolution of these models as well as the lack of observations for urban physical properties. Recently, National Aeronautics and Space Administration (NASA) Earth Observing System (EOS) Moderate Resolution Imaging Spectroradiometer (MODIS) observations illustrate important urban physical properties, including skin temperature, surface albedo, surface emissivity, and leaf area index, It is possible to identify the unique urban features globally and thus simulate global urban processes. An urban scheme is designed to represent the urban-modified physical parameters (albedo, emissivity, land cover, roughness length, thermal and hydraulic properties) and to include new, unique physical processes that exist in urban regions. The urban scheme is coupled with National Center for Atmospheric Research (NCAR) Community Land Model Version 2 (CLM2) and single column coupled NCAR Community Atmosphere Model CAM2/CLM2 to assess the mechanisms responsible for UHI. There are two-steps in our model development. First, satellite observations of albedo, emissivity, LAI, and in situ observed thermal properties are updated in CLM2 to represent the first-order urban effects. Second, new terms representing the urban anthropogenic heat flux, storage heat flux, and roughness length are calculated in the model. Model simulations suggest that human activity-induced surface temperature hazard results in overlying atmosphere instability and convective rainfall, which may enhance the possibility of urban flood hazard.”
“More importantly, although a single urban region may not result in a large impact on global climate, the collective impact of all urban regions on the global climate system is as yet unknown and unstudied. Jin et al. (2005a) show that zonal mean UHI has 1–3 degree warming over the Northern Hemisphere latitudes, implying that the collective UHI may be a significant contributing factor in the overall global warming signal.”
“The current urban scheme does not include potentially important urban land-atmosphere feedbacks, in particular, urban aerosols’ impacts on surface insolation and aerosol-cloud rainfall interactions over urban regions. Therefore, the presented urban impacts are limited to those resulting from changes in the urban surface only. Future model development on coupled urban land-atmosphere interactions is essential for fully understanding the extent of urban impacts.”
The role of urban areas within the climate system is yet another human climate effect whose role was minimized in the 2007 IPCC WG1 Report.